This application claims the priority of German Patent Document 198 44 214.9, filed in Germany on Sep. 26, 1998 and PCT/EP99/06341 filed in Europe on Aug. 28, 1999 the disclosures of which are expressly incorporated by reference herein.
The invention relates to a method for the closed-loop or open-loop control of a forced-induction internal combustion engine.
German Reference DE 40 25 901 C1 has disclosed an exhaust turbocharger for an internal combustion engine that has a turbine with a turbine geometry that can be varied by means of variable turbine guide vanes and a compressor driven by the turbine for increasing the boost pressure in the cylinder inlet. The turbine guide vanes can be adjusted in such a way by means of an actuator that the effective turbine cross section of the turbine is modified. This makes it possible to achieve different exhaust backpressures in the section between the cylinders and the exhaust turbocharger, depending on the operating state of the internal combustion engine, thereby allowing the output of the turbine and the output of the compressor to be adjusted according to requirements. The turbine guide vanes are adjusted to a desired boost pressure in accordance with specified characteristics.
In order to achieve an improvement in efficiency by simple means during nonsteady-state operation of the internal combustion engine, boost-pressure control is performed in accordance with different characteristics above and below a threshold value for the exhaust backpressure. This makes it possible to prevent the occurrence of uncontrolled increases in pressure in the exhaust line upstream of the turbine while the boost pressure is still rising after a positive load change. The internal combustion engine no longer has to expel the exhaust against an increased exhaust backpressure and efficiency is increased.
Another method for closed-loop control of the boost pressure is known from German Reference DE 195 31 871 C1. In order to improve efficiency during nonsteady-state operation of the internal combustion engine, especially after a positive load change from low load and engine-speed ranges, this publication proposes to determine the difference between the exhaust backpressure and the boost pressure as the controlled variable for closed-loop control in order to adjust the boost pressure. This makes it possible to detect an impermissibly high deviation in the exhaust backpressure in the case of a positive load change and to correct it by suitable measures.
The problem underlying the invention is to optimize the operating behaviour of the engine over all load and engine-speed ranges.
With the present method, the engine can be adjusted over the entire operating range with regard to the dual purpose of optimizing fuel consumption and optimizing dynamic response. By distinguishing between the steady-state and the nonsteady-state operating modes, every operating point of the internal combustion engine can be assigned the most favourable characteristic maps for the adjustment of the variable turbine geometry.
The distinction between the different operating modes is made by means of an engine parameter that allows nonsteady operation to be identified. The load gradient and/or the engine-speed gradient are determined or the engine preamble. Nonsteady-state operation is present when the engine parameter exceeds a given limiting value. Below the limiting value, the engine is in steady-state mode.
Nonsteady-state operation and steady-state operation are each assigned a characteristic map, which is used as a basis for adjusting of the variable component of the turbine geometry in order to exert a favourable influence on the operating behaviour of the exhaust turbine with regard to consumption and dynamic behaviour. In this context, the characteristic map assigned to steady-state operation is configured for a low boost pressure and optimum consumption and the characteristic map assigned to nonsteady-state operation is configured for a higher boost pressure and optimum dynamic response. In the configuration for optimum consumption, the air flow rate in the lower load range of the engine is reduced, thereby allowing exhaust and refill losses to be minimized and low fuel consumption to be achieved. This mode of operation is best suited to journeys at approximately constant speed and load, e.g. for open highway travel.
In the configuration for optimum dynamic response, it is advantageous to increase the air flow rate, especially in the lower load range, in order to make available sufficient engine power and, in particular, to enable rapid changes in power. This operating mode is best suited to urban driving conditions.
Once nonsteady-state operation has been detected by means of the engine parameter, it can be maintained for a specifiable holding time. Only after the holding time has expired is the current operating mode expediently reidentified and, depending on the value of the engine parameter, nonsteady-state operation is maintained for a further holding period or a switch made to steady-state operation.
On the other hand, in the characteristic map for optimum consumption, which is assigned to steady-state operation, the variable turbine geometry is expediently held in the open position when the engine speed is low and, at the same time, the load is low, thereby keeping the exhaust backpressure acting on the turbine, the power transmitted to the compressor and the boost pressure at a relatively low level. The exhaust and refill losses are minimized and fuel consumption is reduced.
In the characteristic map assigned to nonsteady-state operation, which is optimized for dynamic response, the variable turbine geometry is advantageously held in the pressure build-up position at low load and low engine speed, in which position the exhaust backpressure upstream of the turbine is increased and the flow of exhaust gas strikes the turbine rotor at a higher speed. This results in a higher boost pressure that has a positive effect on dynamic behaviour.
Adjustment to the desired boost pressure can be performed by closed-loop or open-loop control, a distinction expediently being made between closed-loop and open-loop control by means of further state variables of the engine, in particular by means of the absolute value of the engine speed and load. Thus, for example, open-loop control can be performed at low engine speeds/loads and closed-loop control can be performed at higher engine speeds/loads. Open-loop control at low engine speeds/loads has the advantage of being unaffected by external influences such as falling atmospheric pressure in operation at altitude, which would be superimposed on the boost pressure and would exert an impermissibly great effect in the case of closed-loop control. In the case of closed-loop control operations in the higher engine-speed/load range, the system preferably adjusts to a desired boost-pressure value specified in the characteristic map while, in the case of open-loop control operations, the positioning of the variable turbine geometry is performed in accordance with the values stored in the characteristic map.
It may be expedient to combine closed-loop and open-loop control by first of all adjusting to a rough value for the boost pressure by open-loop control, using the turbine position, and then adjusting to the desired boost-pressure value by closed-loop control for the purpose of fine adjustment. In this case, each operating mode can be assigned two or, if appropriate, more characteristic maps. It is possible to distinguish between the characteristic maps within an operating mode by means of additional engine parameters.